stalling after start

[chaowse]

My 99 explorer started to stall immediatly after I start it. It will rev up to about 2000 than just die. sometimes it will come close to dieing and than rev back up and do this repeatedly until shifted into gear. It will only die if in park or neutral as soon as it gets shifted into any gear than it runs fine. Its been in and out of a shop 4 times and the mechanic is playing with the ignition but it doesnt seem to be helping. I was wondering if anyone had any insight on this problem it would be greatly appreciated